Wireless communications systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user's total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data in assigned channels that correspond to specific unique codes. For example, a mobile station may receive forward channel data signals from a base station that are convolutionally coded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are convolutionally encoded, block interleaved, modulated by a 64-ary orthogonal modulation, and spread prior to transmission by the mobile station. The data symbols following interleaving may be separated into an in-phase (I) data stream and a quadrature (Q) data stream for QPSK modulation of an RF carrier. One such implementation is found in the TIA IS-95 CDMA standard. Another implementation is the TIA S-2000 standard. The order of the Walsh code spreading or 64-ary modulation and PN spreading does not affect the performance of the present invention disclosed below in the DETAILED DESCRIPTION OF THE INVENTION.
A data bit equal to Logic 1 in the convolutionally encoded symbols is transmitted as one 64-chip Walsh code and a data bit equal to Logic 0 is transmitted as the inverse of the 64-bit Walsh code, obtained as an exclusive-OR (XOR) between the data bit and the Walsh code.
In order to increase the reliability of CDMA receivers, base stations and mobile stations frequently transmit M copies of the same signal, staggered in time, to the other device. The receiving device typically uses multiple receive paths, such as in a rake receiver, to capture each of the copies. The captured copies are summed to produce a composite signal in order to improve the signal to noise ratio. This allows the composite signal to be more easily de-spread and recognized by a signal correlator or matched filter. However, this approach requires a large number of components and a large circuit area. Additionally, the repeated transmission of M copies of the same signal is wasteful of scarce bandwidth.
There is therefore a need in the art for improved CDMA systems that have an improved signal-to-noise ratio in the receiver. In particular, there is a need for CDMA systems that do not require the transmission of multiple copies of a signal from a transmitter to a receiver. More particularly, there is a need for an improved CDMA receiver capable of improving the signal-to-noise ratio of a single copy of a received signal.